The present invention relates generally to a power supply for lighting devices. More particularly, the present invention relates to an LED (light emitting diode) driver providing constant output power across a wide range of output voltage and output current.
LED power supplies capable of providing constant output power are known in the art and desirable for, among other reasons, their output flexibility. For example, a 210 W constant power LED driver may be designed to drive LED loads ranging from 1.4 A/150V to 0.7 A/300V. The development time may accordingly be drastically reduced, and product consolidation can be maximized.
Referring to FIG. 1, a conventional example of an input stage for an isolated constant power LED driver 10 may be described. An input rail voltage V1 may be provided from a DC source such as for example the output from a power factor correction (PFC) circuit. The rail voltage V1 is provided as the DC voltage input across a half-bridge switching circuit including switching elements Q1 and Q2. A primary resonant tank is connected to a node between the switching elements Q1 and Q2 to receive an output based on a switching frequency thereof, and includes a primary resonant inductor L1 and resonant capacitor C1. A DC blocking capacitor C2 and a primary winding T1p for an output transformer are coupled in series with each other and across the resonant capacitor C1. In the example shown, output clamping diodes D5 and D6 are provided and limit the output voltage across the resonant capacitor C1 to one half of the rail voltage V1.
There are at least two substantial drawbacks to this solution for constant power LED driver design. One such issue is that it is very difficult to optimize the turns ratio for the output transformer for a wide range of output voltage. A related problem is that it is very difficult to optimize the resonant current going through the resonant inductor for a desired wide range of output current and voltage.
Such problems may be illustrated herein by reference again to the example of a 210 W constant power LED driver. To drive a 300V/0.7 A LED load, the turns ratio of the output transformer will have to be at least 1:1.6 to provide the necessary output voltage. In this case, the resonant inductor current (IL1) reflected on the primary winding T1p of the output transformer will be the output current (0.7 A)×the turns ratio (1.6)=1.12 A. Assuming the same turns ratio for the LED driver design, but now applying a 150V/1.4 A LED load, the resonant inductor current IL1 will be (1.4 A)×(1.6)=2.24 A.
Therefore, the resonant current in the 1.4 A output example is two times higher than that of a 0.7 A load at the same output power level 210 W. This makes the resonant inductor extremely difficult to design, and still further requires the use of switching elements (e.g., MOSFETs Q1 and Q2) with a very high current rating in order to handle the higher end of the current range.
If a dedicated driver is provided for each of 0.7 A and 1.4 A output loads, the output transformer can be optimized so that the resonant current will remain 1.12 A. But optimization of a circuit as shown in FIG. 1 is impossible for constant power and wide-range-output applications because the voltage across the resonant capacitor is clamped.
Referring next to FIG. 2, designers may simply eliminate the clamping diodes (e.g., D5 and D6 from FIG. 1) and optimize the turns ratio of the output transformer for 1.4 A (or the highest output current) to reduce the resonant current. However, the operating frequency for the 0.7 A example will be much higher than is the case when the load is 1.4 A.
As may further be demonstrated in FIG. 3, the frequency difference is primarily a result of the different quality factors (Q) for the resonant tank as corresponding to different LED loads. The natural frequency of the tank is typically high. When the output current is 0.7 A and the output voltage is 300V, the load is: 300V/0.7 A=429 ohms; whereas the load at 1.4 A, 150V is: 150V/1.4 A=107 ohms. As you can see there is four times the difference between load conditions. As a result, the resonant frequency and Q curve will differ greatly with changes in the load. The operating frequency may be very high when the load is 0.7 A, wherein the power converter will conceivably run out of frequency bandwidth in the case where dimming of the LED lighting output is required.
It would therefore be desirable to provide an LED driver circuit for which no large difference was produced on the operating frequency in response to changes in the load at full output, as represented in exemplary fashion in FIG. 4.
It would further be desirable to provide an LED driver circuit for producing a wide range of voltage and current output with constant power driven capability. For example, such an LED driver may desirably enable adjustable output current and output voltage capability across a wide range, for example 1.4 A/150V to 0.7 A, 300V.
It would further be desirable to provide such an LED driver to drive a 210 W lighting array and still further to be dimmable across the lighting range.
It would further be desirable to provide such an LED driver circuit with guaranteed half-bridge soft-switching behavior at all times.
It would further be desirable to provide an LED driver circuit for which no large difference was produced on the resonant current in response to changes in the load.
It would still further be desirable to provide such an LED driver circuit with a self-clamped output.